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Gene therapy for Hemophilia B
About Hemophilia B, or Christmas disease, is an inherited X-linked recessive disorder that results in deficiency of plasma coagulation factor IX. Christmas disease was first described in 1952 (1) in a boy with the last name Christmas. Since hemophilia is an X-linked recessive condition, it occurs predominantly in males. Hemophilia B occurs approximately 1 in 25,000-30,000 males births. Disease state People with hemophilia have spontaneous hemorrhages and excessive hemorrhages in response to trauma, including painful bleeding into joints that causes inflammation and deterioration of the ankles, knees, and elbows. Genetic basis The gene for FIX is located on the long arm of chromosome X, within the Xq27 region. The FIX gene (F9) has 34 kb and composes of 8 exons and 7 intervening sequences. The mature protein has 415 amino acids. Point mutations and deletions in the FIX gene are the causes of hemophilia B (2). Defective factor IX disrupts the intrinsic pathway of the coagulation cascade, preventing clotting and leading to hemorrhages. Treatment The preferred treatment for hemophilia B is replacement of factor IX. However replacement therapy is expensive and the results are not permanent. Even though the recombinant factor IX is safe to use, the goal of hematologists has been the development of successful gene therapy so hemophiliacs can continuously produce endogenous factor IX. In gene therapy a virus containing normal factor IX gene is used to infect patient's cells to correct the genetic disease. Adeno Associated Virus The first virus used as a vector to carry factor IX is the Adeno-associated virus (AAV). THis virus is a good choice for gene therapy because it doesn't cause disease in humans and it infects dividing and quiescent cells. It also doesn't integrate into the genome of the host cell so there is no potental for disrupting normal genes(3). However it was noted that patients infused with AAV-FIX developed cytotoxic T-cell response evidenced by transient trasaminitis. AAV is derived from a wild-type parvovrius and up to 30-50% of the population has been infected with Parvovirus sometime in their lives. People previously infected with Parvovirus have antibodes against the capsid proteins coating the virus, destroying the AAV-FIX when the gene therapy was delivered into circulation. To overcome these problems Dr. Nathwani used another adeno-associated virus, AAV8 for the construct. AAV-8 infects rhesus monkeys not humans so patients have little to immune response to the virus (4). The construct has an expression cassette with the human factor IX gene and is driven by a liver-specific enhancer/promoter LP1. AAV8-FIX is subsequently cross-packaged as a dimer into AAV8 capsid protein and transfected. AAV-based gene therapy vectors form episomal concatamers in the host cell nucleus. In non-divinding cells, these concatemers remain intact for the life of the host cell. Results Using this approach Nathwani et al. demonstrated successful conversion of severe hemophilia B to mild or moderate disease in 6 adults males who underwen intravenous infusion of adeno-associated viral (AAV) vector expressing facot rIX under the control of a liver-restricted promoter. These patients expressed FIX at levels ranging from 1% to 6% of normal for 2 years. Nathwani demonstrated that therapeuctically relevant levels of transduction could be achieved with intravenous infusion as opposed to haptic artery infusion that had been used in the first trial, making the vector delivery procedure much simpler and appealing (5). Potential complications There are several issues that are being studied to make factor IX gene therapy safe and long lasting. For example to avoid immunotoxicity patients with neutralizing antibodies are excluded from clinical trials and liver enzyme levels are monitoryed closely. If they increase the patients are given Prednisone to avoid gene therapy failure. Another problem is the risk of germline transmission, viral DNA has been observed in patients' semen. Another side-effect with lower risk is the chance that patients will devolop liver cancer, which has been observed in mice so trial participants are monitored closely for that possibility as well(6). References 1. Chris Biggs et al. Christmas Disease. British Medical Journal 1952. 2 (4799): 1378–82. 2. http://omim.org/entry/306900 3. Herzog RW et alLong-term phenotypic correction of hemophilia B in a large animal model by AAV-mediated gene transfer.199892Suppl153a Nathwani A C. et al. Self-complementary adeno-associated virus vectors containing a novel liver-specific human factor IX expression cassette enable highly efficient transduction of murine and nonhuman primate liver. Blood. 2006 April 1; 107(7): 2653–2661 5. Nathwani, A. C. et al. Adenovirus-associated virus vector-mediated gene transfer in hemophilia B. New Eng. J. Med. 365: 2357-2365, 2011. 6. High KA. The gene therapy journey for hemophilia: are we there yet?. Blood. Nov 29, 2012; 120(23): 4482–4487.